"Mac is the Retribution's chief engineer. After years of working her way up through the ranks, she rose to the level of Commander with a ship of her own, only to discover that the burden of sending men to their deaths was more than she could handle. She resigned and re-enlisted as a tech. As an Engineering Officer, she was ineligible to take command after Alder's death, leaving that task to the highest ranking Line Officer, Nick Reyes."

Lieutenant Commander Audrey "Mac" MacCallum was Chief Engineer of the Retribution. She was once the captain of her own ship, but resigned her commission after finding she was unable to cope with the responsibility of willingly sacrificing her subordinates, even for the sake of victory.
After Commander Nick Reyes's successful boarding action of the SDF destroyer Ares Vallis, Mac helped to sift the data downloaded by Ethan from its computer, pronouncing it "a gold mine." She also assisted Griff in creating the ATAD drone prior to Reyes's search-and-rescue mission to the mining asteroid over Mercury.
When the Retribution crashed on Mars after the failed attack on the SDF's shipyards, she reproved Reyes for his unwillingness to sacrifice his ship and crew for the sake of completing their mission. She emphasized that she, and every member of the Retribution crew was prepared to give their lives for him, but their deaths would ultimately be meaningless as long as the shipyard remained intact. When Reyes defended his decision, she repeated her belief that an officer unwilling to make the tough decisions has no business being a ship's captain.

Mac was among the small boarding party that Reyes led up the elevator from the surface of Mars to the shipyard. Partway there, they noticed another elevator shaft ferrying reinforcements to the SDF forces on the surface. While Reyes was planting a charge on the next car to descend, the car opened and a C6 robot seized MacCallum. Reyes tried to pull her free before the charge detonated, but she pleaded with him to let her go, for the sake of the mission. Unable to hold on to her, Reyes was helpless to prevent the car from plunging, exploding a short distance down the shaft with Mac and the robots aboard, but blocking the SDF from sending further reinforcements down the shaft. Morosely, Reyes realized that Mac had given her life just to ensure that Reyes and the rest of the party would continue on and complete the mission.